18/05 天声人語

2011年5月18日（水）付

　辞書によれば、〈溶ける〉とは固形物が液状になることだ。類語の〈とろける〉には、心のしまりがなくなる／うっとりする、の意味が加わる。「ろ」が入るだけで随分まろやかだが、間違って「炉」を添えると、〈溶ける〉が牙をむく▼「メルトダウン」は軽々しく使える言葉ではなかった。なにしろ、原子炉の燃料棒が自らの熱で溶け落ちる悪夢である。燃料の損傷という軽い響きも、炉心溶融と正直に書けば、ただならぬ気配に腰が浮く▼福島の事故で、１号機のメルトダウンは津波の数時間後に始まっていたという。東京電力の当初の読みよりずっと早い。隠したなら論外、知らずにいたのならなお怖い。やはり運転中だった２、３号機の燃料も溶けて発熱中らしい▼競うように暴れた末、毒を吐き、枕を並べて熱に浮かされる１～４号機。熱冷ましの注水が汚染水を生み、新たな注水を不要にするための作業には強い放射線が立ちはだかる。放置すれば破局、策を講じたら次なる試練というイバラの道だ▼長く稼いでくれた４兄弟に、安らかな最期をどう用意するか。親にあたる東電が改めた工程表にも、確たる道筋はない。明日をも知れぬ病状と、「来年早々の安楽」の間がどうにも描きづらい▼「孝行息子」が最後にしでかした悪さを前に、政府と東電は保護者の責任を押しつけ合うかのようだ。ここは双方が死力を尽くし、税金と電気料金へのツケ回しを堪（こら）えるのみだろう。国がとろけようかという時に、官も民もない。

17/05 ２・３号機もメルトダウン 東電データで裏付け

2011年5月17日6時12分

　東京電力福島第一原子力発電所の２、３号機でも炉心溶融が起こり、原子炉圧力容器の底に燃料が崩れ落ちるメルトダウンが起きていたとみられることが、１６日に東電が公表したデータで裏付けられた。３号機では溶けた核燃料がさらに下の格納容器内に落ちた恐れもある。専門家は事故直後から指摘しており、細野豪志首相補佐官も１６日の会見で２、３号機でのメルトダウンの可能性を示唆した。

　東電が４月１７日に示した工程表は、６～９カ月で原子炉を安定した状態で停止させるとした。１号機に続き２、３号機でもメルトダウンの可能性が出てきたことにより、工程表の大幅な見直しは必至だ。炉心を冷やすシステムづくりに時間がかかり、高濃度の放射能汚染水の処理も膨大になるからだ。

　今回公表された地震直後のデータは原発内の中央制御室にあり、電源復旧に時間がかかったことや、記録紙に放射性物質が付着しているため整理に時間がかかっていた。公表されたのは、記録紙に打ち出されたグラフや当直長がつける運転日誌などで、大型ファイル４冊分にあたる。

　データによると、圧力容器内の圧力が、２号機は３月１５日午後６時４３分に、３号機は３月１６日午後１１時５０分に、それぞれ下がった。圧力容器の密閉性が損なわれ、圧力が抜けたとみられている。

　圧力容器の底には制御棒や計測機器を外から通すための数多くの貫通部がある。メルトダウンした核燃料が圧力容器の底にたまり、その熱の影響で機器が溶けるなどした結果とみられる。３号機内の汚染水からは、原子炉内の核燃料が損傷して出るテクネチウムなどの放射性物質も確認されていることから、溶けた燃料がさらに圧力容器から格納容器内に落ちた可能性もある。

　東電は会見で「プラント全体の事象を追いかけられておらず、評価できていない」と明確な判断を示さなかった。

　一方、細野氏は会見で炉心に水が入らなかった時間について「１号機は１４時間９分、２号機は６時間２９分、３号機は６時間４３分と短くない」とし「炉心の完全な溶融（メルトダウン）の可能性をみておかないといけない」と話した。

　また原子力安全委員会の班目（まだらめ）春樹委員長は１６日の定例会後の会見で「３月下旬に２号機で高濃度汚染水が発見された時点で、メルトダウンしていたという認識があり、助言した。１号機と３号機も、事故の経緯を考えると同じことが起こっているとの認識を持っていた」と語った。

　東電によると、機器の記録から、運転中だった１～３号機は地震によっていずれも自動停止。配管の破断などの兆候はみられないとしている。非常用ディーゼル発電機も正常に起動していたという。東電はこれらの記録や地震計のデータをもとに、地震直後は機器が正常に作動し、津波到達までは大きな損傷はなかったとみている。（中村浩彦、佐々木英輔）

12/05 Nuclear fuel at Fukushima No. 1 unit melted after full exposure

In this photo released by Tokyo Electric Power Co. (TEPCO), a worker walks on the second floor of the No. 1 reactor building at the Fukushima Dai-ichi nuclear power plant in Okuma town, Fukushima Prefecture, northeastern Japan, early Monday, May 9, 2011.

TOKYO (Kyodo) -- Water inside the troubled No. 1 reactor of the Fukushima Daiichi nuclear power plant was at an unexpectedly low level, not enough to cover the nuclear fuel, hinting that a large part of the fuel melted after being fully exposed, Tokyo Electric Power Co. said Thursday based on data obtained by adjusted gauges.

But the plant operator said the water at the bottom of the reactor pressure vessel holding the fuel is keeping the melted fuel cool, assuring that the company is succeeding in preventing the reactor's fuel from overheating by injecting water from outside.

Based on the latest data after workers adjusted gauges for measuring the water level at the reactor, water could not be confirmed inside the pressure vessel at a point 5 meters below where the top of the 4-meter-long fuel rods normally are, according to the utility also known as TEPCO.

The water level is far lower than earlier thought. Measurements taken before the gauge adjustment indicated that about 1.5 to 1.7 meters of the fuel rods were exposed and not submerged in water.

In this Friday May 6, 2011 image from video footage released on Sunday May 8, 2011 by Tokyo Electric Power Co., workers are seen on the first floor of the turbine building of the Unit 1 reactor building at the crippled Fukushima Dai-ichi nuclear power plant in Okuma town, Fukushima Prefecture, northeastern Japan. (AP Photo/Tokyo Electric Power Co.)

Still, the surface temperature of the pressure vessel was relatively low, measuring between 100 C and 120 C.

The utility had earlier estimated that 55 percent of the reactor core at the No. 1 unit has been damaged. It is unknown how much fuel melted and dropped to the bottom, but the fuel is unlikely to be at its original position.

TEPCO has been trying to check the levels of water inside the No. 1 reactor's pressure vessel and the outer primary container so that it can move ahead with a plan to flood the container with water up to the level above the fuel and create a system to stably keep the fuel cool.

But the latest finding on the situation inside the unit -- one of the six at the plant crippled by the March 11 massive quake and tsunami -- suggests that a significant amount of the water injected into the reactor core to keep the fuel cool as an emergency measure was leaking out to the primary container.

A TEPCO official said the company will review its plan to flood the primary container up to the level above the fuel.

This Sunday, April 10 image taken by T-Hawk drone aircraft and released by Tokyo Electric Power Co. (TEPCO) shows the damaged reactor building of Unit 1 of the tsunami-crippled Fukushima Dai-ichi nuclear power plant in Okuma town, Fukushima Prefecture, northeastern Japan.(AP Photo/Tokyo Electric Power Co.)

Hidehiko Nishiyama, a spokesman for the government's Nuclear and Industrial Safety Agency, said even if the container is not flooded with water, creating a system that would enable coolant to circulate around the reactor is possible by using the water that now exists inside the vessel and primary container.

(Mainichi Japan) May 12, 2011

12/05 １号機、燃料棒すべて落下の可能性と発表

１０日午前、１号機の原子炉建屋内で、水位計の点検をする作業員＝東京電力提供

　東京電力は１２日、福島第一原子力発電所１号機の原子炉圧力容器の水位が、当初の想定より大幅に低く、完全露出した核燃料が、容器底部に落下しているとみられると発表した。

　原子炉を冷やすため水で満たす冠水（水棺）作業が進む格納容器内の水位もはっきりせず、水は漏れだした可能性が高い。東電は「圧力容器の温度は１００～１２０度と安定しているが、冠水作戦は再検討が必要」と説明し、毎時約８トンの注水量を増加させる検討を始めた。

　東電によると、圧力容器の水位は、１０日から原子炉建屋内に入った作業員が水位計を調整して判明。これまで水位は、燃料頂部から約１・６メートル低い位置で事故直後からほとんど変化しなかった。そのため、水位計を調整したところ、燃料頂部から５メートル以上低いことが明らかになった。燃料は長さが約４メートルであることから、完全に冷却水から露出した状態。東電は、既に燃料の大半は溶けたり、崩れたりして、底部に落下したとみている。経済産業省原子力安全・保安院は、圧力容器の温度も低いことから、「燃料は容器底部にたまった水によって冷やされている」と指摘した。

（2011年5月12日13時54分 読売新聞）

13/05 Nuclear fuel rods may have mostly melted

The Yomiuri Shimbun

Fuel rods in the No. 1 reactor at the Fukushima No. 1 nuclear power plant may have mostly melted and dropped to the bottom of the pressure vessel as the water level in the vessel has been found to be significantly lower than thought, Tokyo Electric Power Co. said Thursday.

The fuel rods in the reactor are believed to be completely exposed, according to TEPCO, the operator of the plant.

So-called water entombment operations to fill the containment chamber with water are continuing in an effort to cool the reactor. But the water level in the chamber cannot be clearly determined, and water is likely leaking from it, the utility said.

TEPCO said the temperature in the pressure vessel is stabilized at 100 C to 120 C but that the water-entombment plan, in which water was expected to be filled to about 1 meter above the top of the fuel rods, needs to be reconsidered. The company is considering increasing the amount of water injected into the pressure vessel, which currently stands at about 8 tons per hour.

TEPCO learned about the water level of the pressure vessel after workers who entered the reactor building beginning Tuesday adjusted a water-level gauge. Previously, the reading of the water level had remained almost unchanged at about 1.6 meters below the top of fuel rods since immediately after the outbreak of the crisis at the plant following the March 11 earthquake and tsunami.

After adjusting the gauge, workers found the actual water level was more than 5 meters below the top of the fuel rods. As the fuel rods are about 4 meters long, they are considered to have been fully exposed above the cooling water, TEPCO said.

It said it believes the fuel rods mostly have fallen to the bottom of the pressure vessel after melting or collapsing.

The Economy, Trade and Industry Ministry's Nuclear and Industrial Safety Agency said the fuel has been cooled with water pooled at the bottom of the pressure vessel as the temperature of the vessel remains relatively low.

At the bottom of the steel pressure vessel, which is 16 centimeters thick, the water level is believed to stand at a maximum of only about 4 meters, TEPCO said. The company believes that most of the 190 tons of water injected every day is leaking from the pressure vessel, which is likely to be damaged more seriously than previously thought.

More than 10,000 cubic meters of water had been injected into the reactor as of Thursday, exceeding the combined 360-cubic-meter capacity of the pressure vessel and the 7,800-cubic-meter capacity of the containment chamber.

It is highly likely that water is leaking from both the pressure vessel and containment chamber and flowing into underground parts of the reactor building and the adjacent turbine building, TEPCO officials said.

To proceed with the water-entombment, it is necessary to accurately grasp the water level in the containment chamber and the conditions of pipes that take in or release cooling water, experts said.

(May. 13, 2011)

20/03 Workers must cool 4,546 spent fuel rod bundles

2011/03/20

More than 4,500 spent fuel rod bundles must be cooled in six reactors at the Fukushima No. 1 nuclear power plant, according to information released by Tokyo Electric Power Co.

A total of 4,546 bundles of fuel rods are stored in the six reactors, with each bundle containing several dozens of fuel rods, which in turn hold hundreds of fuel pellets.

The spent fuel rods release large amounts of heat. And since the cooling systems at the reactors were damaged in last week's Great East Japan Earthquake and subsequent tsunami, there is the danger of damage to the fuel rod bundles from overheating if water in the storage pools evaporates.

The amount of heat being emitted from the No. 4 reactor is especially high. A total of 2 million kilocalories per hour is being emitted, about three times the combined heat coming from the No. 1 to No. 3 reactors.

In addition to 783 spent fuel rod bundles, 548 bundles that have not been completely used are stored at the No. 4 reactor. Those unused fuel rods had been removed from the core in order to replace equipment.

The large amount of heat being released from the No. 4 reactor is due to the fact that the fuel rods not completely used emit more heat than spent fuel rods.

The storage pool at the No. 4 reactor has a capacity of about 1,400 cubic meters. Based on the level of heat being emitted from the fuel rods, a simple calculation shows that the water temperature in the pool rises about 2 degrees every hour.

If the cooling mechanism can be repaired to resume operations, the water temperature can be maintained at under 40 degrees.

But if the tsunami damage makes the cooling mechanism inoperable, it would take about a day for water in the pool to begin boiling due to the heat from the fuel rods. It would take an additional 10 days or so to completely evaporate all the water in the pool.

TEPCO employees said they saw what looked like water in the storage pool of the No. 4 reactor from a helicopter that flew over the plant on Thursday.

The spent fuel rods stored at the other reactors emit much lower levels of heat. The level emitted depends on the capacity of the storage pool and the number of fuel rod bundles stored.

The storage pool at the No. 3 reactor holds about half of the capacity for fuel rod bundles. Efforts to spray water on the No. 3 reactor from firefighting trucks and helicopters have continued since Thursday.

Because the No. 3 reactor only has 514 fuel rod bundles stored in its pool, the amount of heat emitted from the fuel rod is only about 10 percent of the heat from the No. 4 reactor.

However, white smoke has been spewing since Wednesday from the damaged outer building housing the No. 3 reactor, so TEPCO workers have been unable to confirm the condition of the storage pool.

The No. 5 reactor has the second-highest level of heat emitted from spent fuel rods. While the spent fuel rods take up about two-thirds of the pool capacity, the amount of heat emitted is only one-third that from the No. 4 reactor.

Unlike the No. 1 to No. 4 reactors, the pump that circulates water to the storage pool still works in the No. 5 reactor.

The water temperature in the pool was maintained at 66.3 degrees as of 2 p.m. Friday.

However, the cooling mechanism at the No. 5 reactor does not work due to the power outage caused by the earthquake.

Although a restoration of the cooling mechanisms could lower the water temperatures in the storage pools, there is still the question of the cooling mechanisms for the reactor cores.

One problem that TEPCO may be facing is malfunctioning monitors.

Data about water levels in the pressure containers of the cores released by TEPCO indicate that about one-half to one-third of the length of the fuel rods in the core is exposed above water.

However, even though seawater has continued to be pumped into the No. 1 reactor core, the water level data has not changed at all, indicating the monitoring equipment may have malfunctioned.

TEPCO workers have no way of confirming if the equipment is working properly.

Moreover, readings of the pressure within the suppression pool under the containment vessel in the No. 2 reactor have stopped since Tuesday. The suppression pool may have been damaged by an explosion Tuesday morning.

While the reason is unknown, readings of the pressure within the suppression pool in the No. 3 reactor have also stopped since Monday night.